The densities of NH and NH 2 radicals in an Ar-NH 3 plasma jet created by the expanding thermal plasma source were investigated for various source-operating conditions such as plasma current and NH 3 flow. The radicals were measured by cavity ringdown absorption spectroscopy using the ͑0,0͒ band of the A 3 ⌸ ← X 3 ⌺ − transition for NH and the ͑0,9,0͒-͑0,0,0͒ band of the à 2 A 1 ← X 2 B 1 transition for NH 2 . For NH, a kinetic gas temperature and rotational temperature of 1750± 100 and 1920± 100 K were found, respectively. The measurements revealed typical densities of 2.5 ϫ 10 12 cm −3 for the NH radical and 3.5ϫ 10 12 cm −3 for the NH 2 radical. From the combination of the data with ion density and NH 3 consumption measurements in the plasma as well as from a simple one-dimensional plug down model, the key production reactions for NH and NH 2 are discussed.